C-shaped magnetic resonance imaging system

ABSTRACT

An open C magnet system for magnetic resonance imaging comprises circular NdFeB poles, a single piece yoke having beveled inside and outside faces between the vertical post section and the horizontal arms of the yoke, a necked-in mid-section of the vertical post, flat Neodymium corner plates in face to face relationship with the inside beveled faces between the vertical post and the horizontal arms.

FIELD OF THE INVENTION

[0001] This invention relates to the design of permanent magnetstructures, and more particularly it relates to an Open-C shapedpermanent magnet design.

BACKGROUND OF THE INVENTION

[0002] Magnetic resonance imaging (“MRI”) is used primarily in medicalsettings to produce high quality images of the inside of the human body.MRI is based on the principles of nuclear magnetic resonance (“NMR”), aspectroscopic technique used by scientists to obtain information aboutthe microscopic chemical and physical characteristics of molecules. MRIstarted out as a tomoographic imaging technique, producing an image ofthe NMR signal as a thin slice through the human body. MRI has nowadvanced beyond tomographic imaging to become a volume imagingtechnique.

[0003] The MRI image is created when the nuclei of atoms are placed in amagnetic field and are exposed to non-ionizing radio frequency (“RF”)energy at a specific frequency. An RF pulse is emitted that excites thenuclei away from equilibrium. When the pulse is switched off, the nucleireturn to their original state and in the process emit energy at RFfrequencies. The signal is picked up by a receiver coil and is convertedinto images through the application of a sophisticated mathematicalalgorithm.

[0004] An MRI system comprises a main magnet, a gradient system. RFcoils, a transmitter, a receiver and a computer equipped with imagingsoftware. The magnets in an MRI system may be superconducting magnets,electromagnets or permanent magnets.

[0005] A permanent magnet system differs from superconducting magnetsystems in that superconducting systems require significant amounts ofelectricity and liquid helium for maintaining coolant circulation, airconditioning systems, and for the various electronics components. Thesite requirements for superconducting systems are also more demandingthan for permanent magnet systems due to the specific needs of thecryogen.

[0006] Electromagnets also require cooling (usually by water) andproduce a relatively weak magnetic field.

[0007] Strong magnetic fields, low eddy currents and a high degree ofhomogeneity of the magnetic field are critical to producing good qualityMRI images. Most prior art MRI systems using permanent magnets arelimited in the strength of the magnetic fields they can produce havingregard to the magnetic materials generally available to produce themagnets.

[0008] Japanese Patent 08045729 to Sumitomo Special Metals Co. Ltd.,dated Feb. 16, 1996 discloses a magnetic field generating device for anMRI incorporating a C-shaped yoke.

[0009] Chinese Patent 94115507.2 to Dong et al., dated Apr. 24, 1996discloses an Open-C shaped permanent magnet design. The production modelof the magnet was made with poles made of NdFeB. While this material hasvery good magnetic properties, it is very expensive. As such it is notcommonly used for large permanent magnets. The cost of the NdFeB used inthe Dong et al. design remains high.

[0010] The Dong et al. design uses octagonal poles, pole pieces andrings. However the octagonal shapes produce regions of high flux densityat the corners of the octagons, which distorts the shape of the field inthe imaging volume (the gap between the poles). Ideally, the horizontalcross-section of the field in the gap is perfectly circular. It is notpossible to achieve this with the octagonal design without carefulshimming to correct the field distribution after the magnet ismanufactured. The Dong et al. design is therefore limited in thehomogeneity of magnetic field it can achieve, and suffers thedisadvantage of requiring elaborate shimming and having high eddycurrents.

[0011] The Dong et al. design also uses a stepped shape of corner piecesof magnetic material (steel) located at the inside corners of the “C”.This was done to decrease the amount of metal and thus reduce the weightand cost of the magnet. However, this results in some areas having toomuch material removed and some too little. This causes saturation in thesteel and increases the amount of leakage flux and the size of the fieldaway from the magnet. For safety reasons, this requires a larger room.

[0012] The outer edges of the pole piece and rings on the Dong et al.magnet extend vertically from the pole. The function of these pieces isto improve the uniformity of the field in the gap, but if improperlydesigned, they reduce the efficiency of the magnet thus requiring alarger than necessary amount of NdFeB to be used to obtain the desiredfield strength.

[0013] In addition, a significant amount of time is needed to shim theDong et al. magnet after it is manufactured. The Dong et al. pole pieceswere made of steel which is desirable to improve the homogeneity of thefield, however it has very low resistively which allows eddy currents toform when an MRI scan is being performed. Eddy currents reduce thesignal to noise ratio of the system, resulting in poorer images. TheDong et al. magnet was created out of a number of steel plates that hadto be machined to the proper dimensions, and bolted together. Thisincreased the cost of each magnet significantly. Furthermore, since theyoke is bolted together, there is an increased risk that the magneticforce between the poles of the magnet could be stronger than the boltstrength holding the yoke arms parallel. To eliminate this risk,vertical columns needed to be added between the yoke arms as additionalsupport members. This partially obstructed the gap between the polescausing difficulties in positioning the patient bed in the gap duringscans.

[0014] It is an object of the present invention to provide an Open-Cshaped, NdFeB-based permanent magnet design that minimizes the amount ofNdFeB required, but that nonetheless provides good homogeneity of fieldin the gap, minimizes the need for shimming and minimizes eddy currents.

SUMMARY OF THE INVENTION

[0015] This invention is a permanent open C-type magnet.

[0016] In one of its aspects, the invention comprises a magneticresonance imaging magnet system comprising a generally C-shaped yokeformed from a single piece of cast steel. The yoke has a vertical post,diagonal sections extending from each end of the post, the diagonalsections having angled inside and outside faces, and a substantiallyhorizontal arm extending from each of the diagonal sections. A pair ofpoles is supported on the arms, the poles being made substantially ofNdFeB and having pole faces in opposed and spaced relation to define animaging volume between them. The post has a portion of reduced width atits medial extent. The ends of the arms adjacent the imaging volume arealso beveled inward on the opposite side from the imaging volume. Asubstantially flat magnetized NdFeB plate is mounted on the inside faceof each of said diagonal sections in face to face relationship pith saidinside face, the inside face and the plate defining an angle ofapproximately 45 degrees in relation to the post, and the plates arepoled to oppose leakage flux between the nearest pole and the post. Acomposite pole plate of magnetic material is mounted on each of the polefaces. Each of the pole plates comprises a tape wound laminated thinsheet of silicon steel with an insulating adhesive between the layers,the laminated sheet being segmented into a plurality of wedge-shapedelements formed into a circular disk shape with an insulating adhesivebetween the respective elements. Each of the pole plates is covered witha layer of high resistivity material and a pole ring is mounted on eachof the pole plates.

[0017] In another aspect of the invention, the imaging volume has avertical extent of at least 0.47 m, the diameter of the poles is between1.0 and 1.2 m, and the thickness of the pole plates is between 45 and 60mm. The post has a vertical extent of between 1.8 and 1.9 m, the widthof the diagonal sections is between 1.2 and 1.4 m, and the depth of thepost is between 1.7 and 1.9 m.

[0018] The Open-C shaped permanent magnet according to the inventionproduces 0.35 T mid-field strength that is much better than the otherpennanent magnet designs available in market.

[0019] The invention will be more fully appreciated by reference to thedetailed description of the preferred embodiment that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The preferred embodiment will be described by reference to thedrawings thereof in which:

[0021]FIG. 1 is a perspective view of the magnet structure according tothe preferred embodiment of the invention;

[0022]FIG. 2 is a top view of the magnet structure;

[0023]FIG. 3 is a front view of the magnet structure;

[0024]FIG. 4 is a side view of the magnet structure;

[0025]FIG. 5 is a perspective view illustrating the general arrangementof the pole piece; and

[0026]FIG. 6 is a perspective view illustrating the general arrangementof the pole ring.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0027]FIG. 1 is a perspective view illustrating the overall appearanceof the magnet structure embodying the present invention. The maincomponents of the magnet are the yoke 1, the pole 2, corner NdFeB magnetpieces 3, the pole plate 4, and the rings 5.

[0028] The yoke 1 is generally C-shaped. This is preferable overstandard tunnel systems or other open systems in terms of patientcomfort and patient accessibility for doctors and radiologists since itis open on three sides. The open C-shaped design also provides for thepossibility of MRI guided interventional surgery which is of significantfuture importance. The yoke has a vertical post and two horizontal armsextending from the post. The magnet poles 2 depend from the ends of thearms 10. Pole plates 4 and rings 5 are provided on the faces of thepoles 2.

[0029] The gap between opposed rings 5 that define the imaging volume ispreferably 0.47 m or greater. With respect to the yoke 1, its height ispreferably 1.83 m, its width at its widest part is preferably 1.31 m,and its depth is preferably 1.8 m. The diameter of the pole 2 ispreferably 1.11 m. It is contemplated that these dimensions could varysuch that the diameter of the poles is between 1.0 and 1.2 m, and thethickness of the pole plates is between 45 and 60 mm. The post may havea vertical extent of between 1.8 and 1.9 m, and the width of thediagonal sections may be between 1.2 and 1.4 m. The depth (thickness) ofthe post may be between 1.7 and 1.9 m.

[0030] The size of the magnet, the gap size, and the large homogeneousfield volume produced are such that it allows the magnet to be used as awhole body scanning system.

[0031] As can be seen in FIG. 1, on the inside faces of the arms 10 ofthe yoke 1 are mounted two oppositely poled magnet elements or poles 2which are of NdFeB, a permanently magnetized material. The facingsurfaces of the poles 2 are provided with pole plates 4, and rings 5.These are made of a material suitable to shape the magnetic field in thecenter of the gap into a very homogeneous distribution ideal for MRI.The characteristics of the material used in the pole plates 4 and therings 5 will be further discussed below.

[0032] Corner NdFeB magnet plates 3 are mounted on the inside angledfaces of the yoke 1 in face to face relationship with the inside angledcorners of the yoke, as seen in FIG. 4. The magnet plates 3 are poled tooppose the tendency to establish a return path from the arms and thepoles to the post of the yoke. This improves the homogeneity of themagnetic field in the imaging volume.

[0033] The yoke 1 serves two main functions, namely to provide a lowreluctance return path for the magnetic field, and to provide mechanicalsupport for the magnet poles 2. The precise shape of the yoke shown inFIGS. 1-4 was designed to give improved magnetic performance for MRIapplications while allowing as open an area for the patient as possible.Among the characterizing features of the design are the angled faces 12,14, 16, 18, 20, and 22 on the top and bottom, front and back as shown inFIG. 4, the necked-in section 24 best seen in FIGS. 1 and 3, and themajor dimensions referred to above mentioned. All of these serve tomaintain functionality and efficiency while reducing the weight of themagnet as much as possible.

[0034] The yoke 1 is also formed from a single cast piece of steel. Thishas significant consequences. It reduces the cost of machining andassembling large individual pieces of plate steel, which is veryimportant when production of many identical units is desired. It alsoensures that all the units produced will be of the same quality sincethey are all made from the same mold. Finally, it improves thestructural integrity of the yoke. Since the magnet poles stronglyattract one another, and the weight of he top pole is not insignificant,the arms 10 of the yoke 1 must be able to withstand these forces andprevent the magnet poles 2 from pulling together, which is a significantsafety issue. In fact, the yoke must be strong enough so that the armsremain parallel. If they were to deflect even slightly, the homogeneityof the field in the gap would be compromised. Casting the yoke in asingle piece and in the shape shown ensures that the strength of theyoke is more than sufficient to prevent the magnet poles from comingtogether, and that they remain parallel. In a C-shape configuration,this is not as easy to achieve with a yoke made of individual piecesbolted together without using additional support struts, etc., which caninterfere with the openness of the design. In other permanent magnetdesigns, either a C-shape yoke is not employed, but rather 4 pillars areused equally spaced around the magnet poles, or a C-shape with twosupport pillars has been used.

[0035] The corner Neodymium (NdFeB) pieces 3 are located on the insideangled faces of the yoke as shown in FIG. 1. Since the yoke size wasreduced as much as possible, as explained previously, this brings themagnet poles 2 close to the vertical back section of the yoke. Normally,this would cause the field in the gap to be distorted since some of theflux that should travel vertically between the two poles would prefer to‘short’ back to the yoke. This would adversely affect the homogeneity.To compensate for this, the corner elements 3 are used and are mountedat a 45-degree angle as shown, to be most effective in preventing thedistortion of the field. To prevent this distortion without the cornermagnet elements, the yoke arms would need to be extended significantlyso that the magnet poles 2 would be far from the vertical section of theyoke. However his would greatly increase the weight of the yoke (becauseof all the added material) and would complicate the problem of keepingthe arms parallel. The volume of the permanent magnet material used ineach corner is optimized to ensure that the field in the gap is ashomogeneous as possible, and for this design is preferably 8000 cubiccm.

[0036] The pole plate 4 and rings 5 shown in FIG. 1 are very importantto controlling the uniformity of the field in the gap as mentionedpreviously. The pole plate must be of a certain thickness to essentiallyaverage out the field produced by the permanent magnet material of themagnet pole on which it is mounted, however, it cannot be too thick thatit significantly reduces the intensity of the field. For this design,the thickness of pole plate 4 is preferably 53 mm. The function of therings 5 is to increase the volume of the region of the homogeneous fieldin the gap, and its height and width are carefully optimized for this.For this design the cross-section of rings 5 is preferably 67 mm highand 83 mm wide. These elements are preferably made of steel or anothermaterial with similar magnetic characteristics to properly shape thefield without reducing the field strength, or saturating which reducesthe efficiency of the magnet and increases the leakage flux.

[0037] One other important characteristic of the material used for thepole plate 4 and the rings 5 is the electrical resistivity. Using plainsteel or iron for these components, during a MRI scan, would result ineasy generation of eddy currents in the elements so as to causeartifacts in the image. To reduce this effect in this magnet, thematerial used for the pole plate and rings is a thin strip of siliconsteel that has been folded over itself and laminated together.

[0038] The elements of the pole plate laminated material are cut intopie-shaped sections and reassembled as shown in FIG. 5 and FIG. 6 inorder to prevent eddy currents from travelling in complete circularpaths around the elements. The selection of the material and producingsuch a laminated assembly is crucial to the performance of the magnetsince both the magnetic properties and the electrical properties areimportant and it is not easy to optimize them simultaneously.

[0039] The pole plate 4 is constructed by ‘tape winding’ in a spiral along, thin strip of silicon steel, and applying an insulating epoxybetween successive layers of the spiral to hold it together. In thismanner, the plate is constructed to the proper outer dimensions. Oncethe adhesive has cured, the top and bottom of the plate are ground andetched to ensure that the plate is of the proper thickness. This plateis then cut into several wedge-shaped sectors to prevent eddy currentsfrom circulating around the winding. These pieces are then rejoinedagain using an insulating epoxy. This results in a solid plate which isthen machined with proper mounting holes for the ring, and the magnetyoke. The ring is also produced in a similar fashion using a tape woundlamination. It is cut into several sections, and bonded together andonto the top of the laminated pole plate. We have found that this mannerof producing laminated and composite pole plates effectively reduces theeddy currents.

[0040] The material used is silicon steel which has a higher resistivitythan other steel, which is important in reducing eddy currents. However,the effect of the laminations on the magnetic field is not good comparedto solid metal. For this reason, the stacking factor should be as highas possible, which requires a very thin metal strip for winding and goodprocess control.

[0041] It will be appreciated that certain modifications may be made tothe preferred embodiment described above without departing from theprinciples of the invention.

1. A magnetic resonance imaging magnet system comprising: a generallyC-shaped yoke (1) formed from a single piece of cast steel; said yoke(1) having a vertical post, diagonal sections extending from each end ofsaid post, said diagonal sections having angled inside and outsidefaces, and a substantially horizontal arms (10) extending from each ofsaid diagonal sections; a pair of poles (2) supported on said arms (10),said poles (2) being made substantially of NdFeB and having pole facesin opposed and spaced relation to define an imaging volume between them;said post having a portion of reduced width at its medial extent; theends of said arms (10) adjacent said imaging volume being beveled inwardon the opposite side from the imaging volume; a substantially flatmagnetized NdFeB plate (3) mounted on the inside face of each of saiddiagonal sections in face to face relationship with said inside face,said inside face and said plate (3) defining an angle of approximately45 degrees in relation to said post, said plates (3) being poled tooppose leakage flux between the nearest pole (2) and the post; a pair ofcomposite pole plates (4) of magnetic material, one mounted on each ofsaid pole faces, each of said pole plates (4) comprising a tape woundlaminated thin sheet of silicon steel with an insulating adhesivebetween the layers, said laminated sheet being segmented into aplurality of wedge-shaped elements, said wedge-shaped elements beingformed into a circular disk shape with an insulating adhesive betweenthe respective elements; each of said pole plates (4) being covered alayer of high resistivity material; and, a pole ring (5) mounted on eachof said pole plates (4).
 2. A magnet system as in claim 1 wherein saidimaging volume has a vertical extent of at least 0.47 m, the diameter ofsaid poles (2) is between 1.0 and 1.2 m, and the thickness of said poleplates (4) is between 45 and 60 mm.
 3. A magnet system as in claim 2wherein the post has a vertical extent of between 1.8 and 1.9 m, thewidth of said diagonal sections is between 1.2 and 1.4 m, and the depthof said post is between 1.7 and 1.9 m.